As is well known in the art, it is very important to pay a particularly attention to the contrast of a reproduced image so as to make the image clear or sharp. More specifically, to increase the contrast of the reproduced image, it is necessary to drive a picture tube by a video signal with a sufficiently large voltage. The maximum voltage, however, should be less than a threshold voltage at which undesirable blooming takes place.
In the following, there will be discussed an inherent defect of prior art in connection with FIGS. 1-3. In FIG. 2(A), reference character a denotes a waveform of a video signal indicating a white level. As is previously referred to, the white level should be less than a threshold value E1 at which the blooming takes place. In the prior art, in order to increase the sharpness of a reproduced image, the video amplifier is designed so as to have a frequency characteristic as shown in FIG. 1, so that the resulting video signal has an overshoot as best shown in FIG. 2(B). However, in FIG. 2(B), when the video signal is reduced to a 100% white level, the overshoot exceeds the threshold voltage E1, the resulting in the reproduced image being deteriorated by the blooming. To avoid this defect, the peak of the overshoot is conventionally set in such a manner as to be less than the threshold voltage E1. However, this decreases undesirably the contrast of the reproduced image.
It has been experimentally confirmed that the appropriate magnitude of the overshoot is about 30% of the video signal. However, the magnitude of the overshoot should be carefully determined in order not to adversely affect the contrast of the reproduced image. This has been hitherto a problem in the circuit design of a video amplifier. Furthermore, in the prior art, a TV viewer cannot easily adjust a reproduced image to a desirable extent due to the blooming.
In order to remove the above mentioned drawback inherent in the prior art, a method has been proposed that the video signal is clamped at the threshold voltage E1. This proposal, however, is accompanied by some shortcomings. That is, although any trouble is not invited when the low frequency component of the video signal (corresponding to the signal in FIG. 2(A)) does not exceed E1 as shown in FIG. 3(A), the video signal will be sliced, or distorted as shown in FIG. 3(B) when the degree of modulation of the transmitted video signal is varied at random by an error which may be present in the transmitter.